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Abstract:

A time-multiplexed thermal sensing circuit is provided for control and
sensing of two thermistors over a single line electrical coupling. The
circuit may include a first diode that selectively couples or isolates a
first thermistor from a sense node based on the polarity of an applied
voltage to the sense node. The circuit may further include a second diode
that selectively couples or isolates a second thermistor from the sense
node based on the polarity of the applied voltage to the sense node such
that only of the thermistors is coupled to the sense node at any time.

Claims:

1. A thermal sensing circuit, said circuit comprising: a first
thermistor; a first diode coupled between said first thermistor and a
sense node, said first diode configured to isolate said first thermistor
from said sense node in response to a positive voltage applied to said
sense node; a second thermistor; and a second diode coupled between said
second thermistor and said sense node, said second diode configured to
isolate said second thermistor from said sense node in response to a
negative voltage applied to said sense node.

2. A circuit according to claim 1, wherein the anode of said first diode
is coupled to a first port of said first thermistor, the cathode of said
first diode is coupled to said sense node and a second port of said first
thermistor is coupled to a ground.

3. A circuit according to claim 2, wherein the anode of said second diode
is coupled to said sense node, the cathode of said second diode is
coupled to a first port of said second thermistor and a second port of
said second thermistor is coupled to said ground.

4. A circuit according to claim 1, wherein said first thermistor and said
second thermistor have negative temperature coefficients.

5. A circuit according to claim 1, wherein said thermal sensing circuit
is disposed in a thermal substrate.

6. An LED-based lamp assembly comprising: a thermal substrate comprising
a first thermistor, a first diode, a second thermistor and a second
diode, said first diode configured to isolate said first thermistor from
a sense node in response to a positive voltage applied to said sense
node, said second diode configured to isolate said second thermistor from
said sense node in response to a negative voltage applied to said sense
node; an LED-based light source disposed on said thermal substrate; and a
driver circuit configured to drive said LED-based light source, said
driver circuit comprising: a control circuit configured to select one of
said first thermistor and said second thermistor; a multiplexer circuit
configured to apply a control voltage to said sense node, wherein the
polarity of said control voltage is based on said control circuit
selection; and a sensing circuit configured to monitor the voltage at
said sense node and determine a temperature associated with said selected
thermistor.

7. An LED-based lamp assembly according to claim 6, wherein said
multiplexer circuit and said sensing circuit share a common electrical
coupling to said sense node.

8. An LED-based lamp assembly according to claim 6, wherein said driver
circuit adjusts power to said LED-based light source based on said
determined temperature.

9. An LED-based lamp assembly according to claim 6, wherein said
determined temperature is calculated based on pre-determined calibration
data, said calibration data relating thermistor voltage and temperature.

10. An LED-based lamp assembly according to claim 6, wherein said sensing
circuit comprises a microprocessor.

11. An LED-based lamp assembly according to claim 6, wherein said control
circuit comprises a microprocessor.

12. An LED-based lamp assembly according to claim 6, wherein the anode of
said first diode is coupled to a first port of said first thermistor, the
cathode of said first diode is coupled to said sense node and a second
port of said first thermistor is coupled to a ground.

13. An LED-based lamp assembly according to claim 12, wherein the anode
of said second diode is coupled to said sense node, the cathode of said
second diode is coupled to a first port of said second thermistor and a
second port of said second thermistor is coupled to said ground.

14. An LED-based lamp assembly according to claim 6, wherein said first
thermistor and said second thermistor have negative temperature
coefficients.

15. An LED-based lamp assembly according to claim 6, wherein said sensing
circuit monitors said voltage at said sense node biased to a positive
voltage range.

16. A method for controlling and sensing thermistors, said method
comprising: applying a positive voltage to a sense node during a first
time period, wherein said positive voltage application selectively
couples a first thermistor to said sense node through a first diode and
selectively isolates a second thermistor from said sense node through a
second diode; applying a negative voltage to said sense node during a
second time period, wherein said negative voltage application selectively
isolates said first thermistor from said sense node through said first
diode and selectively couples said second thermistor to said sense node
through said second diode; monitoring the voltage at said sense node;
determining a temperature associated with said first selectively coupled
thermistor based on said monitored voltage during said first time period;
and determining a temperature associated with said second selectively
coupled thermistor based on said monitored voltage during said second
time period.

17. A method according to claim 16, further comprising calculating said
determined temperature based on pre-determined calibration data, said
calibration data relating thermistor voltage and temperature.

18. A method according to claim 16, further comprising adjusting power to
an LED-based light source based on said determined temperature.

Description:

TECHNICAL FIELD

[0001] The present application relates to time-multiplexed operation of
dual thermistors, and in particular, to time-multiplexed operation of
dual thermistors using a single line electrical coupling for control and
sensing of the thermistors.

BACKGROUND

[0002] The development of high-brightness LEDs has led to use of such
devices in various applications and lighting fixtures. In general, an
LED-based lamp operates in a fundamentally different way than an
incandescent, or gas discharge lamp, and therefore may use a driver
circuit tailored to deliver power to the LEDs according to their
requirements. The driver circuitry for an LED-based lamp generally
converts an alternating current (AC) input, such as a 120V/60 Hz line
input to a stable direct current (DC) voltage used for driving the
LED-based lamp. In some applications, for example automotive
applications, the driver circuitry converts a DC input, such as from a
12V battery, to a stable DC voltage at a different level to drive the
LED-based lamp.

[0003] The LEDs are often mounted on a thermal substrate, which in turn is
electrically coupled to the driver circuit that may be located on a
separate module. In addition to the mounting circuitry, the thermal
substrate may include one or more thermistors used to monitor temperature
and provide feedback to the driver for the purpose of limiting or
regulating power, and thus temperature, to a desired operational range.
In many applications, the connector between the driver module and the
thermal substrate is of a pre-defined type or specification and the
number of connector pins may be limited. In these cases it can be
difficult to accommodate the extra electrical connections that would
typically be needed to monitor a second thermistor.

[0004] One solution to this problem is to employ an additional, or
intermediate, circuit board assembly on the thermal substrate to excite
and monitor multiple thermistors and then transmit a composite signal
back to the driver module over the single available electrical line to
the driver module. This approach, however, suffers from the disadvantages
of additional cost, complexity and size.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] Reference should be made to the following detailed description
which should be read in conjunction with the following figures, wherein
like numerals represent like parts:

[0006]FIG. 1 is a block diagram of one exemplary embodiment of a system
consistent with the present disclosure;

[0007]FIG. 2 is a block diagram of another exemplary embodiment of a
system consistent with the present disclosure;

[0008]FIG. 3 is a block diagram of one exemplary embodiment of a dual
multiplexed thermistor circuit consistent with the present disclosure;

[0009]FIG. 4 is a schematic diagram of one exemplary embodiment of a dual
multiplexed thermistor circuit consistent with the present disclosure;
and

[0010]FIG. 5 is a block flow diagram of one exemplary method consistent
with the present disclosure.

DETAILED DESCRIPTION

[0011] Generally, this disclosure provides circuits and methods for
implementing time-multiplexed operation of dual thermistors over a single
electrical line or coupling. The thermistors, along with associated
selection circuitry, may be located on a thermal substrate for an
LED-based lamp. The multiplexer and sensing circuitry may be located on a
separate module along with the LED driver circuit. The electrical
coupling between the driver module and the thermal substrate may provide
a single line for the purpose of control and sensing of the two
thermistors. The selection circuitry on the thermal substrate may include
two diodes, each diode associated with one of the thermistors. The diodes
may be configured to selectively isolate or couple the associated
thermistor to a shared sense node based on the polarity of a control
voltage applied to the sense node. The control voltage may be generated
by the multiplexer circuitry on the driver module and the sense node may
be monitored by the sensing circuitry on the driver module.

[0012] Turning now to FIG. 1, there is provided a simplified block diagram
of one exemplary embodiment of a system 100 consistent with the present
disclosure. In general, the system includes a control/sensing circuit 102
consistent with the present disclosure, for selecting, exciting and
sensing thermistors. The control/sensing circuit 102 may be coupled to
two thermistors: thermistor 1 106 and thermistor 2 108. The coupling may
be achieved through a multiplexer circuit 104 that enables the
control/sensing circuit 102 to electrically couple to the thermistors
106, 108 through a single, shared electrical line.

[0013] Thermistors are electrical components that exhibit a variable
resistance that is dependent on temperature. By exciting a thermistor
with a known voltage and then measuring or sensing the current, the
resistance of the thermistor may be determined and thus the temperature
measured. Alternatively, a known current may be applied and the voltage
can be measured to determine the resistance of the thermistor. Some
thermistors may have positive temperature coefficients, where the
resistance increases as the temperature increases, while other
thermistors may have negative temperature coefficients, where the
resistance increases as the temperature decreases. Some thermistors
operate in a linear mode, where the relationship between resistance and
temperature may be linear or approximately linear. Some thermistors
exhibit a more complicated relationship between resistance and
temperature, in which case a predetermined calibration curve may be
employed to determine temperature based on a resistance measurement.

[0014]FIG. 2 is a block diagram of another exemplary embodiment of a
system consistent with the present disclosure. As shown, an LED-based
lamp assembly 202 may include an LED driver module 204 and a thermal
substrate 206 upon which a number of LED(s) 208 may be disposed. In
general, the LED driver module 204 may include circuitry to convert an
alternating current (AC) input, such as a 120V/60 Hz line input to a
stable direct current (DC) voltage used for driving the LED(s) 208,
although other configurations are also possible. The LED driver module
204 may also include the control/sensing circuit 102 and the multiplexer
circuit 104. The thermal substrate 206 provides a heat dissipation
platform for the LED(s) 208, and may also include circuitry including the
thermistors 106 and 108 as well as other components to be discussed in
greater detail below. The thermistors 106, 108 may be monitored to
determine the temperature of the thermal substrate 206 and/or the LED(s)
208. In some embodiments, the level of power delivered by the LED driver
module 204 to the LED(s) 208 may be adjusted based on the monitored
temperature to maintain the system within a desired operational
temperature range.

[0015] The electrical connection between the LED driver module 204 and the
thermal substrate 206 may include electrical lines or pins, the number of
which may be limited. The multiplexer circuit 104 allows for control and
sensing of the two thermistors 106, 108 over a single line or pin 210, as
will be described in greater detail below, resulting in a more efficient
utilization of a the limited available connections.

[0016]FIG. 3 is a circuit block diagram 300 that conceptually illustrates
one exemplary embodiment of a dual multiplexed thermistor circuit
consistent with the present disclosure. The control/sensing circuit 102
is shown to include a separate control circuit 302 and a sensing circuit
304 for clarity of description although there is no requirement that
these components be separated. In some embodiments, the control circuit
302 and the sensing circuit 304 may be implemented as a microprocessor
which may further include an analog-to-digital converter.

[0017] Also shown is the multiplexer circuit 104 which includes a positive
voltage source 306, a negative voltage source 308 as well as controllable
switches 310 and 316 and resistors 312 and 314. In some embodiments, the
controllable switches 310 and 316 may be transistors such as, for
example, bipolar junction transistors (BJTs) or field effect transistors
(FETs). The thermal substrate 206 is also shown to include the
thermistors 106 and 108, diodes 318 and 320 and a sense node 322. The
multiplexer circuit 104 is coupled to the sense node 322 of the thermal
substrate 206 by the single electrical coupling line 210.

[0018] The control circuit 302 may be configured to select either of the
thermistors 106, 108 for excitation and measurement by selectively
coupling either the positive voltage source 306 or the negative voltage
source 308 to the sense node 322 by operation of either of the
controllable switches 310 or 316 respectively. For example, when the
switch 310 is closed (in a conducting state) and the switch 316 is open
(in a non-conducting state), a positive voltage may be applied to the
sense node 322. Under this condition, the diode 318 may isolate the
thermistor 106 from the sense node 322, while the diode 320 may couple
the thermistor 108 to the sense node 322, effectively selecting the
thermistor 108 for excitation and measurement. In this situation, the
resistor 312 and the thermistor 108 may operate as a voltage divider
between the positive voltage source 306 and a ground. Measurement of the
voltage, VSENSE, at the sense node 322 by the sensing circuit 304
allows for the determination of the resistance of the thermistor 108, and
thus the temperature associated with the thermistor 108.

[0019] Alternatively, when the switch 310 is open (in a non-conducting
state) and the switch 316 is closed (in a conducting state), a negative
voltage may be applied to the sense node 322. Under this condition, the
diode 318 may couple the thermistor 106 to the sense node 322, while the
diode 320 may isolate the thermistor 108 from the sense node 322,
effectively selecting the thermistor 106 for excitation and measurement.
In this situation, the resistor 314 and the thermistor 106 may operate as
a voltage divider between the negative voltage source 306 and the ground.
Measurement of the voltage, VSENSE, at the sense node 322 by the
sensing circuit 304 allows for the determination of the resistance of the
thermistor 106, and thus the temperature associated with the thermistor
106.

[0020] Thus, the selection of a thermistor for excitation as well as the
measurement of the selected thermistor, during a given time period, may
be accomplished using a single electrical connection, the line 210. In
some embodiments, additional pairs of thermistors may be deployed on the
thermal substrate 206 and each additional pair of thermistors may be
coupled to the multiplexer circuit 104 through a separate additional
electrical connection line.

[0021]FIG. 4 is a schematic diagram of one exemplary embodiment of a dual
multiplexed thermistor circuit 400 consistent with the present
disclosure. The illustrated exemplary embodiment includes the multiplexer
circuit 104, thermal substrate 206, a capacitor C1, diodes D1-D3,
transistors Q1-Q5, resistors R1-R9, a switch S1 and thermistors Z1 and
Z2. In operation, the control circuit, which in some embodiments may be a
microprocessor (not shown), toggles the switch S1 to couple either the +5
volt power rail (for positive mode operation) or the ground (for negative
mode operation) to the multiplexer circuit 104 to control the operation
of the multiplexer circuit 104 by driving the gates (or bases) of the
transistors Q1, Q2 and Q4 either high or low. The control circuit may
also measure the voltage at the VUPROCESSOR node. In some
embodiments, this may be accomplished with an analog-to-digital
converter.

[0022] When operating in the positive mode, the base of the transistor Q1
is driven high which switches Q1 into a non-conducting state, which in
turn drives the gate of the transistor Q5 low, switching Q5 into a
non-conducting state, decoupling resistor R7 from the circuit. The gate
of the transistor Q2 is also driven high in the positive mode, switching
it into a non-conducting state, which effectively decouples the resistor
R5 from the circuit. The base of the transistor Q4 is also driven high
which switches Q4 into a conducting state, which in turn drives the gate
of the transistor Q3 low and thus switches Q3 into a conducting state.
This combination of the transistors Q2 and Q5 in a non-conducting state
with the transistor Q3 in a conducting state creates a path from the +5
volt power rail through the resistor R4 to the sense node 322, which
applies a positive voltage at the node. The positive voltage at the sense
node 322 causes the diode D2 to isolate the thermistor Z1 from the sense
node 322 while allowing the diode D3 to couple the thermistor Z2 between
the sense node 322 and the ground. This effectively creates a voltage
divider R4/Z2 between the +5 volt power rail and the ground. Since the
value of the R4 resistor and the value of the forward voltage drop of the
D3 diode over the temperature range are known, a measurement of the
voltage at the sense node 322 enables a determination of the resistance
of the thermistor Z2, and therefore a determination of the associated
temperature. Equivalently, the control circuit may measure the voltage at
the VUPROCESSOR node, which is coupled to the sense node 322 through
the resistor R6 of known value. The value of the thermistor Z2 operating
in the positive mode may be estimated as:

[0023] When operating in the negative mode, the base of the transistor Q1
is driven low which switches Q1 into a conducting state, which in turn
drives the gate of the transistor Q5 high, switching it into a conducting
state. The gate of the transistor Q2 is also driven low in the negative
mode, switching it into a conducting state, which creates a series
resistance network consisting of the resistors R5, R6 and R7 between the
+5 volt power rail and the -5 volts power rail. The base of the
transistor Q4 is also driven low which switches Q4 into a non-conducting
state which in turn drives the gate of the transistor Q3 high and thus
switches Q3 into a non-conducting state. This combination of the
transistors Q2 and Q5 in a conducting state with the transistor Q3 in a
non-conducting state creates a path from the -5 volt power rail through
the resistor R7 to the sense node 322, which applies a negative voltage
at the node. The negative voltage at the sense node 322 causes the diode
D3 to isolate the thermistor Z2 from the sense node 322 while allowing
the diode D2 to couple the thermistor Z1 between the sense node 322 and
the ground. This effectively creates a voltage divider R7/Z1 between the
-5 volt power rail and the ground. Since the value of the R7 resistor is
known, a measurement of the voltage at the sense node 322 enables a
determination of the resistance of the thermistor Z1, and therefore a
determination of the associated temperature. Equivalently, the control
circuit may measure the voltage at the VUPROCESSOR node, which is
coupled to the sense node 322 at the junction of the resistors R5 and R6.
The resistors R5 and R6 may be operable as a voltage divider configured
to bias or level shift the sense node voltage, which is in a negative
voltage range, to the VUPROCESSOR node voltage in a positive voltage
range that may be more conveniently measured by the control circuit
analog to digital converter. The value of the thermistor Z1 operating in
the negative mode may be estimated as:

[0024] A dual multiplexed thermistor circuit 400 consistent with the
present disclosure may be configured for operation with a variety of
input voltages based on appropriate selection of various circuit
components thereof. Table 1 below identifies one example of circuit
components useful in configuring the embodiment 400 illustrated in FIG. 4
(resistor values in ohms):

[0025] In a preferred embodiment, the dual multiplexed thermistor circuit
400 has input power rails of +/-5 volts and thermistors Z1, Z2 having
negative temperature coefficients.

[0026]FIG. 5 is a block flow diagram of one method 500 for controlling
and sensing two thermistors consistent with the present disclosure. The
illustrated block flow diagram may be shown and described as including a
particular sequence of steps. It is to be understood, however, that the
sequence of steps merely provides an example of how the general
functionality described herein can be implemented. The steps do not have
to be executed in the order presented unless otherwise indicated.

[0027] In the exemplary embodiment illustrated in FIG. 5, a positive
voltage is applied to a sense node during a first time period 502. The
positive voltage application selectively couples a first thermistor to
the sense node through a first diode and selectively isolates a second
thermistor from the sense node through a second diode. A negative voltage
is applied to the sense node during a second time period 504. The
negative voltage application selectively isolates the first thermistor
from the sense node through the first diode and selectively couples the
second thermistor to the sense node through the second diode. The voltage
at the sense node is monitored 506. A temperature associated with the
first selectively coupled thermistor based on the monitored voltage
during the first time period is determined 508. A temperature associated
with the second selectively coupled thermistor based on the monitored
voltage during the second time period is determined 510.

[0028] Consistent with the present disclosure, therefore, there are
provided circuits and methods for implementing time-multiplexed operation
of dual thermistors over a single electrical line or coupling. The
thermistors, along with associated selection circuitry, may be located in
a thermal substrate for an LED-based lamp. The multiplexer and sensing
circuitry may be located on a separate module along with the LED driver
circuit. The electrical coupling between the driver module and the
thermal substrate may provide a single line for the purpose of control
and sensing of the two thermistors.

[0029] According to one aspect of the disclosure there is provided a
thermal sensing circuit. The thermal sensing circuit includes: a first
thermistor; a first diode coupled between the first thermistor and a
sense node, the first diode configured to isolate the first thermistor
from the sense node in response to a positive voltage applied to the
sense node; a second thermistor; and a second diode coupled between the
second thermistor and the sense node, the second diode configured to
isolate the second thermistor from the sense node in response to a
negative voltage applied to the sense node.

[0030] According to another aspect of the disclosure there is provided an
LED-based lamp assembly including: a thermal substrate comprising a first
thermistor, a first diode, a second thermistor and a second diode, the
first diode configured to isolate the first thermistor from a sense node
in response to a positive voltage applied to the sense node, the second
diode configured to isolate the second thermistor from the sense node in
response to a negative voltage applied to the sense node; an LED-based
light source disposed on the thermal substrate; and a driver circuit
configured to drive the LED-based light source, the driver circuit
comprising: a control circuit configured to select one of the first
thermistor and the second thermistor; a multiplexer circuit configured to
apply a control voltage to the sense node, wherein the polarity of the
control voltage is based on the control circuit selection; and a sensing
circuit configured to monitor the voltage at the sense node and determine
a temperature associated with the selected thermistor.

[0031] According to another aspect of the disclosure there is provided a
method for controlling and sensing thermistors, the method including:
applying a positive voltage to a sense node during a first time period,
wherein the positive voltage application selectively couples a first
thermistor to the sense node through a first diode and selectively
isolates a second thermistor from the sense node through a second diode;
applying a negative voltage to the sense node during a second time
period, wherein the negative voltage application selectively isolates the
first thermistor from the sense node through the first diode and
selectively couples the second thermistor to the sense node through the
second diode; monitoring the voltage at the sense node; determining a
temperature associated with the first selectively coupled thermistor
based on the monitored voltage during the first time period; and
determining a temperature associated with the second selectively coupled
thermistor based on the monitored voltage during the second time period.

[0032] As used in any embodiment herein, "circuitry" may include, for
example, singly or in any combination, hardwired circuitry, programmable
circuitry, state machine circuitry, and/or firmware that stores
instructions executed by programmable circuitry.

[0033] The term "coupled" as used herein refers to any connection,
coupling, link or the like by which signals carried by one system element
are imparted to the "coupled" element. Such "coupled" devices, or signals
and devices, are not necessarily directly connected to one another and
may be separated by intermediate components or devices that may
manipulate or modify such signals. Likewise, the terms "connected" or
"coupled" as used herein in regard to mechanical or physical connections
or couplings is a relative term and does not require a direct physical
connection.

[0034] While the principles of the invention have been described herein,
it is to be understood by those skilled in the art that this description
is made only by way of example and not as a limitation as to the scope of
the invention. Other embodiments are contemplated within the scope of the
present invention in addition to the exemplary embodiments shown and
described herein. Modifications and substitutions by one of ordinary
skill in the art are considered to be within the scope of the present
invention, which is not to be limited except by the following claims.